1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger in which the engine operation can be changed between a "one-turbocharger-operation," wherein only the first turbocharger is operated, and a "two-turbocharger-operation," wherein both the first turbocharger and the second turbocharger are operated.
2. Description of the Prior Art
As illustrated in FIG. 10 (Prior Art), a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. An exhaust switching valve 17' is installed in an exhaust conduit 20b' downstream of the turbine 8a'. Similarly, an intake switching valve 18' is installed in an intake conduit 14b' downstream of the compressor 8b'. Further, an intake bypass valve 33' is installed in an intake bypass conduit 13' bypassing the compressor 8b'. The intake switching valve 18' and the exhaust switching valve 17' are closed at small to medium intake air quantities to produce a "one-turbocharger-operation" wherein only the first turbocharger 7' is operated. The intake switching valve 18' and the exhaust switching valve 17' are opened at large intake air quantities to produce a "two-turbocharger-operation" wherein both the first turbocharger 7' and the second turbocharger 8' are operated.
Japanese Patent Publication SHO 59-145328 discloses changing the engine operation between the "one-turbocharger-operation" and the "two-turbocharger-operation" on the basis of an intake air quantity only. For reference, FIG. 6 illustrates making this change only on the basis of an intake air quantity. Further, Japanese Patent Publication SHO 60-259722 discloses changing the engine operation between the "one-turbocharger-operation" and the "two-turbocharger-operation" on the basis of an engine speed only. For reference, FIG. 7 illustrates making this change only on the basis of an engine speed.
However, in the case of changing the engine operation only on the basis of an intake air quantity, as illustrated in FIG. 8, whenever a gear position change of the transmission occurs during acceleration, the intake air quantity Q, which corresponds to the engine speed NE, momentarily decreases and causes the engine operation to change from the "two-turbocharger-operation" to the "one-turbocharger-operation". During subsequent acceleration, the "one-turbocharger-operation" quickly changes to the "two-turbocharger-operation" and a sudden decrease in the charging pressure occurs. This decrease in the charging pressure causes a torque shock and degrades the drive feeling. This torque shock occurs frequently during acceleration and the durability of the switching valves is decreased. In contrast, a change from the "two-turbocharger-operation" to the "one-turbocharger-operation" is not accompanied by such a decrease in the charging pressure and a great torque shock.
Where the operational change between the "one-turbocharger-operation" and the "two-turbocharger-operation" is made only on the basis of an engine speed, the changes between the "one-turbocharger-operation" and the "two-turbocharger-operation" will occur frequently during periods of constant operation. A driver will keenly feel torque shocks which occur at the operational changes, and therefore, the drive feeling is degraded with the durability of the switching valves being decreased.